Non-Newtonian shear viscosity and first normal stress coefficient of polymer melts

NON-NEWTONIAN SHEAR VISCOSITY AND FIRST NORMAL STRESS COEFFICIENT OF POLYMER MELTS 

The most obvious viscoelastic phenomenon in polymer melts is the decrease of viscosity with increasing shear rate. This decrease may amount to several decades. At the same time, elastic behaviour may be observed. 

9 (left) Increase of shear stress after starting steady shear flow at zero time for constant shear rate in the Newtonian region, (right) Time dependent shear viscosity, rj+[t) from Newtonian to non-Newtonian behaviour. 

Upon starting a steady shear flow with constant shear rate yQ = q the shear stress for a Maxwell element increases gradually in time to a steady state value rjq according to (see Chap. 13) 

A number of empirical equations have been proposed to describe the influence of shear rate on viscosity. The most popular equation represents the so-called power law formulated by Ostwald (1925) and De Waele (1923)  

---